Research On Mechanism Of Fracture Initiation And Propagation In Shale With Defects Hydraulic Fracturing

Shale gas is an abundant unconventional natural gas and the highest reserves in the world are China.Hydraulic fracturing is a technology to increase production of shale gas which is widely developed and applied currently.One of the criteria for judging effectiveness is the ability to form complex hydraulic fracturing fracture networks.There are many defects such as joints,bedding,cracks,micro-holes and micro-cracks inside,which makes the expansion of hydraulic cracks inside shale extremely complicated.Therefore,it has important theoretical value and guiding significance by hydraulic fracturing to realize large-scale and commercial development of shale gas for researching crack initiation and propagation law of hydraulic fractures in shale containing defects.It carry out the evaluation of compressibility before fracturing,the prediction of yield after fracturing and the mechanism of improving the low permeability.This paper analyzes the mechanical characteristics,failure characteristics and ultrasonic characteristics of defective shale during loading and damage.A constitutive model of defect-bearing shale damage is established.The paper discusses the criteria for intersecting hydraulic fractures and defects,and establishes a numerical model for hydraulic fracturing of shale with bedding and natural fractures.According to the analysis for hydraulic fracturing extension path,failure mode,intersection mode and influencing factors of shale with bedding and natural fractures,the main results and innovations show as follows:1.The formation characteristics of hydraulic fracture network with layered shale are discussed by experiments and numerical simulations.The network is mainly composed of cracks along the weak bedding plane and cracks in the direction of the maximum principal stress,including secondary fractures in other directions.The crack along the direction of the maximum principal stress weakens the friction along the bedding plane and promotes the accelerated expansion along the bedding crack.At the same time,the existence of the difference in the ground stress causes the hydraulic crack in the bedding direction to be closed by the extrusion.2.The paper establishes a numerical model for the intersection of hydraulic fractures and multi-level natural fracture clusters.And the effects of natural crack length,hydraulic crack and natural crack approach angle and incident flow rate on the intersection and extension of hydraulic cracks and natural crack clusters are analyzed.The results show that the hydraulic crack intersects the natural crack and the natural crack opens and expands along its bilateral along to the larger approach angle,the bigger horizontal stress difference and the longer crack length.With the decrease of approach angle,horizontal stress difference and the natural crack length,the hydraulic crack intersects the natural crack by opening the natural crack and bilaterally expanding to open and unilaterally expanding,and finally becoming directly through the natural crack expansion.3.By analyzing the shale failure mode after shale hydraulic fracturing with bedding and natural fracture defects.The results show that hydraulic fractures' crack usually in the form of mixed failure and are mainly shear-damaged at the end of the perforation.At the branch initial end,it usually splits in a mixed failure manner and turn along the bedding direction.At the crack intersection,it is still cracked in a mixed manner and then spreads forward in a shear failure manner.When the angle and in-situ stress difference is large,the initial failure mode of the hydraulic crack and the natural crack intersects the tensile failure,and the process damage mode is dominated by the mixed failure mode.On the contrary,the length of the natural crack is reduced,and the cracks expand into a gradual and unilateral expansion.Finally it becomes a direct expansion through the natural crack.4.Based on the existing extended forms of hydraulic fractures and natural fractures,the hydraulic crack and the natural crack intersect and expand and further improve and refine by numerical simulation.It mainly increases the unilateral opening of natural cracks and expands and opens natural cracks,expands the simultaneous bifurcation and so on.Finally,the hydraulic cracks and natural cracks intersect and expand into 10 cases.5.In the numerical simulation,the method of global embedding of cohesive elements is applied to the crack initiation and expansion of complex shale hydraulic fractures with bedding and natural fractures.The results show that it could better simulate the cracking,extension,steering,bifurcation,closure,intersection and other complex behaviors,so that the simulation is closer to the actual production situation.In this paper,the simulation method of cohesive elements can obtain the intersection failure mode and intersection expansion mode between hydraulic fractures with bedding and natural fractures.It can provide some reference and guidance for the study of hydraulic fracturing technology and related theoretical research.The paper has 107frames,15 tables,and 138 references.